In a transmitter used in a base station of a radio communications system, an amplifying device having high power efficiency is required in order to miniaturize the transmitter and reduce its power consumption. As technologies to realize high efficiency of the amplifying device, technologies are known in which the power consumption can be reduced by controlling a power supply voltage depending on an input signal. A particularly effective system of them is an Envelope Tracking system.
The Envelope Tracking system is a system in which a power supply voltage of an amplifier such as a power amplifier and the like is changed depending on the amplitude of an input signal. Since the electric power supplied from a power supply decreases by this system in comparison with driving it by a constant power supply voltage, it becomes possible to amplify signals high efficiently. Here, if an output signal voltage exceeds the power supply voltage of the power amplifier, the linearity between the input signal and the output signal is lost due to the output saturation arising. Therefore, in general, the power supply voltage is designed so that the power supply voltage supplied to the power amplifier may always become higher than the output signal voltage.
Meanwhile, since low distortion characteristics are generally required for the amplifying device used in the transmitter, a distortion compensation system such as Digital Pre-distortion (DPD) system and the like is adopted. The digital pre-distortion system is a system in which the distortion caused by the power amplifier is cancelled by means of preliminarily distorting signals input into the power amplifier composing the amplifying device using a pre-distorter and consequently the low distortion characteristics of the amplifying device are realized. In the digital pre-distortion (DPD) system, a distortion model of the power amplifier is constructed by comparing an input signal and an output signal of the power amplifier, and inverse characteristics of the distortion model are set for the pre-distorter.
Configuration examples of the digital pre-distortion (DPD) system include a system using a look up table (LUT). A correction value corresponding to each of the amplitudes of the input signals has been preliminarily recorded in the look up table (LUT), and the correction value is updated sequentially so that the distortion of the power amplifier may be cancelled. There are two kinds of distortion which are compensated by using the look up table (LUT). One is a distortion due to non-linear characteristics in output amplitude characteristics to input amplitudes of the signal (hereinafter, referred to as “AM-AM characteristics”) and another is a distortion due to non-linear characteristics in output phase characteristics to input amplitudes (hereinafter, referred to as “AM-PM characteristics”). By correcting two kinds of distortion mentioned above, signal characteristics between the input signal and the output signal of the amplifying device become a linear relationship and consequently the distortion is suppressed. Here, the look up table (LUT) is not limited to the above-mentioned configuration which separately includes one for correcting the AM-AM characteristics and another for correcting the AM-PM characteristics. A system is also known which configures a look up table (LUT) including complex values by using input-output data stream of complex signals.
Configuration examples of the digital pre-distortion (DPD) system include another system approximating a correction amount of distortion for the power amplifier by using a polynomial. In this case, the non-linear characteristics of the power amplifier are cancelled by calculating coefficient values of the polynomial using the output signal and the input signal of the power amplifier, and by updating the coefficient values sequentially. Another system is also known in which the pre-distorter has filter characteristics, and a memory effect of the power amplifier is suppressed taking the past information into consideration by selecting the number of taps of the filter.
Although each amplifying device individually adopting either the Envelope Tracking system or the digital pre-distortion (DPD) system has been described above, it is also possible to configure an amplifying device which adopts both systems. An example of the above-mentioned amplifying device is disclosed in patent literature 1.
As shown in FIG. 12, a related amplifying device 500 described in patent literature 1 includes a power amplifier 510, a power supply modulation unit 520, a distortion compensation unit 530, a first estimation unit 540, and a second estimation unit 550. The power supply modulation unit 520 supplies the power amplifier 510 with a power supply modulation voltage depending on an input signal, and the distortion compensation unit 530 compensates the distortion in the power amplifier 510. The first estimation unit 540 estimates a first amplifier model which indicates the characteristics of the power amplifier 510 between input and output signals on the basis of the input signal and the output signal of the power amplifier 510. The second estimation unit 550 estimates a second amplifier model which indicates the characteristics of the power amplifier 510 between the power supply modulation voltage and the output signal on the basis of the power supply modulation voltage and the output signal. The distortion compensation unit 530 compensates the distortion of the power amplifier 510 on the basis of the first amplifier model, and the power supply modulation unit 520 controls the power supply modulation voltage on the basis of the second amplifier model.
As mentioned above, in the related amplifying device 500, the distortion compensation unit 530 and the power supply modulation unit 520 control independently the distortion compensation and the power supply modulation voltage on the basis of the first amplifier model and the second amplifier model, each of which is obtained by the estimation unit independently of each other. Therefore, it is said that it becomes possible to control suitably both of power supply modulation characteristics and distortion compensation characteristics depending on the characteristics of the power amplifier 510, and that it is possible to simplify the calculation for carrying out these controls.    Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 2010-045508 (paragraphs [0014] and [0015], and FIG. 3)